I installed a NIB Redhead ASP 1.08 ABC with a Bisson Pitts muffler in my Rare Bear. I'd estimate level flight speed at about 95MPH and I haven't had any problems with my stabs with about 10 flights. However I've installed 3-ply Birchply doublers in the horizontal stabs and blocking for Robart Hingepoints. In this thumbnail pic, you can see how the first rib was completely cut through to the hole for the Fiberglass Spar. Also note how the trailing edge has been split for a length of about 4 inches. This cut severly compromises the structural integrity of the horizontal stab and is most likely the root cause of most of the tail failures we've been reading about.

Another thing to consider which hasn't been brought up is the basic airframe rigging (Trim) as it's setup by TT. I measured the wing incidence and engine trust line of my RB during construction and didn't care for the amount of downthrust that was built into the firewall. IIRC (my measurements are in a much earlier post), There was about 4.5 degrees of downthrust and 2 degrees of right thrust. I reduced the engine downthrust to 1.5 degrees and subsequently added washers to arrive at a 3 degree setting. I reduced the right thrust to 1 degree and it's been about right.

I mention this because one has to take into consideration what the horizontal stab really does in flight. To get right to the point, the horizontal stab is developing a downforce to counteract the effects of the aerodynamic center of lift. The force vector is located behind the aircraft's static CG, this results in a nose down rotation around the aircraft's lateral axis (imagine a line running spanwise through the CG). To counteract this force a stabilizing force is required, or a downforce, this is what the horizontal stab does.

Now what happens when you add downthrust to the firewall ?? We induce a nose down rotation around the aircraft's lateral axis. This is another downforce that the horizontal stab must now counteract. The horizontal stab is counteracting sum of the downforce vector induced by the aerodynamic center of pressure, and downthrust vector induced by the firewall downthrust.

Now let's consider what we'rre doing when we add a BIG BLOCK engine like an O.S. 1.60 for it's weight. This engine is going to create much more thrust than a TT .91 engine will. Now consider what what's happening aerodynamically to the airframe to balance the large increase in engine downthrust at full power. Obviously the horizontal stab must counteract this increased nose down force with a corresonding taildown force. This is increasing the loads on the horizontal stab. One can assume that some point we might exceed the design limits of the horizontal stab.

Mix the above scenario , with suspect workmanship present in the TT Rare Bear horizontal stab, one can only expect in flight failure of the tail.

Just my $.02.

Rich